The existence of single-wall magnetic domains in particular materials under certain conditions has been reported in the literature as well as in many patents. The potential for using the single-wall domain or bubble devices for information storage and logic functions for electronic computers and the like has also been studied. The primary goal in using magnetic bubble devices is the same as that for using any new material, i.e., reducing unit cost without sacrificing performance. Some of the problems foreseen in using magnetic bubble devices are similar to the problems encountered in conventional data processing components such as the problem of connecting a component in a system to other similar cooperating components. Since cost is related to size many commercial electronic data processing components have been reduced to almost microscopic size, such as integrated circuits. This small size, although a blessing from the cost standpoint, causes problems in making connections to the components. Similarly, bubble devices must be provided with signals for control, data read-in and data readout. In the design and fabrication of any such bubble device, of course, the manner of providing these connections must be taken into account.
A majority of the bubble devices that have been proposed in the past have been two dimensional, that is, essentially planar. However, it is generally understood that three-dimensional information storage is a more economical and efficient type of storage device. For instance, most of the bubble devices now being experimented with require an in-plane rotating magnetic field for purposes of propagation. The magnetic field acting in a thin patterned permalloy layer creates magnetic poles causing the bubbles to propagate in preferred directions as determined by the pattern. However, the rotating magnetic field in such arrangements is only effective in the thin layer in which the permalloy lies. If the bubble devices could be made three-dimensional the information density could be increased without increasing the equipment necessary to generate the rotating magnetic field. In line with the goal of decreasing costs a prime requirement is to increase the density of information storage. One method of increasing the information density is to decrease the bubble diameter. However, as the diameter of the bubbles is decreased, the size of the control lines which are necessary for selective bubble generation and bubble annihilation must also decrease. The difficulty in this approach lies in the fact that the current carrying requirement of these control lines is not decreased. A rule of thumb, employed in the art, is that above a current density of 10.sup.6 A/Cm.sup.2 electromigration may occur which is capable of physically destroying the conductors. This obviously provides at least one limit to the state of the art in achievable information density.
Another method of increasing the amount of information is to merely increase the area occupied by the device. As has been pointed out above, however, a rotating magnetic field is required for propagation purposes in most magnetic bubble devices. Increasing the area occupied by the memory increases the size of the coils necessary to generate this magnetic field. As a result, increasing the area occupied by the memory does not decrease the per bit storage cost.